Preclinical research on the functional involvement of the endogenous opioid system in mediating alcohol consumption has led to investigations of the effectiveness of the opioid antagonist naltrexone for the treatment of alcohol dependence. The specific aims of this study are to: - determine individual differences in endogenous opioid systm activity as measured by naloxone challenge test, mu opioid receptor binding as measured by PET, serum 6-b-naltrexol levels as measured by gas chromatography mass spectrometry, and subjective reports of alcohol craving during a 2-week inpatient protocol. - determine drinking and related psychosocial outcomes during a 12-week randomized clinical trial of naltrexone treatment for alcohol dependence comparing thr4ee medication conditions: placebo, once daily naltrexone 100 mg, or twice daily naltrexone 50 mg. All patients will receive standardized Motivational Enhancement Therapy. - determine the utility of endogenous opioid system activity, mu opiod receptor binding, serum 6-b-naltrexol levels, and subjective reports of alcohol craving as predictors of alcohol consumption levels (e.g., elapse rates and quantity/frequency of drinking) and other clinical outcomes during outpatient naltrexone treatment.